In general, the production of a metal by using an apparatus for a molten salt electrolysis is performed by electrolysis for oxidizing and reducing a metal salt in a molten state on the surface of an electrode pair. The apparatus for molten salt electrolysis is designed so that the heat balance is maintained by taking into consideration heat generated from the electrode pair during the electrolysis process and heat insulation of the electrolytic cell. In addition, the electrolysis process is operated incorporating ways and means so as to eliminate thermal disturbance generated in case of supplying a molten salt into the apparatus for molten salt electrolysis during the electrolysis. However, there may be the case where a temperature of the molten salt turns into a tendency of the descending or ascending due to various factors. In the case where the temperature of the molten salt descends, a part of the molten salt is solidified, and continuation of the electrolysis is impossible, and therefore, the heating of the molten salt is required. Conversely, in the case where the temperature of the molten salt ascends, a re-reaction between electrolyzed metal and produced gas increases to cause a descending of the current efficiency, and therefore, the cooling of the electrolytic cell becomes necessary.
In addition, the heating of molten salt is required, at the time of a starting up of the production of metal. Here, the terms “at the time of starting up of the production of metal” mean the time immediately after charging a molten salt, prepared in a separate vessel, into the electrolytic cell. At this point, the molten salt is in contact with the wall surface of the electrolytic cell, whereby some amount of the heat in the molten salt is removed, and therefore, heating for the molten salt up to the working temperature becomes necessary. In an extreme case, there is a concern that the molten salt is partially solidified between a pair of electrodes, thereby causing a situation where normal electrolysis is not able to be performed.
Under the foregoing circumstances, there have been proposed various technologies regarding the temperature control of the molten salt in the apparatus for molten salt electrolysis.
For example, as disclosed in PTLs 1 and 2, there is a known method in which a heat exchanger with a built-in gas burner is installed in an electrolytic cell of an apparatus for molten salt electrolysis, and electrolysis is performed while controlling heating or cooling by the heat exchanger such that a molten salt is kept in the completely molten state.
However, in order that at the time of starting up of the electrolytic cell, the molten salt is heated and kept in the completely molten state only by the heat exchanger with a built-in gas burner before it is solidified, it is necessary to install a heat exchanger equipped with a considerably large number of gas burners in the electrolytic cell, and hence, such state is not economical.
In addition, as disclosed in PTL 3, there is also a known means for supplying gas which has been pre-heated in another unit of an electrolytic cell into an interior of the electrolytic cell thereof, which means thereby heating a molten salt.
However, moisture formed as a by-product of gas combustion is contained in the combustion gas produced in another unit, and therefore, if this gas is carried into the electrolytic cell, not only an electric power is consumed for the electrolysis of water from the moisture absorbed into the molten salt, but also an electrode is oxidized by an oxygen gas produced by the water electrolysis, and thus, an undesirable phenomenon may arises.
In this way, in the method for producing metal by using apparatus for molten salt electrolysis, in particular, a method for efficiently heating the molten salt, the electrolysis process is desired without causing inconvenience.